It is often desirable when viewing images obtained from volumetric medical image data to view simultaneously images for the same patient or other subject. The anatomy displayed in each image may be largely identical, but the pathology or other properties of the anatomy may differ.
The different images may be obtained from scans or other measurements on the patient or other subject performed at different times, such that there may be differences in a pathology or other condition between the images as the pathology or other condition develops over time. Alternatively, the different images may be obtained from multiple phases of the same set of scans taken during a single measurement procedure, for example images of parts of the respiratory, cardiac or vascular systems at different times during a respiratory or cardiac cycle.
In each case, there may be movement of anatomical structures between the phases or times when the images were obtained. In the case of images obtained at different phases of a respiratory, cardiac or vascular cycle, there may be expansion or contraction of different anatomical structures.
It can be desirable for a radiologist or other operator to be able to navigate synchronously through the different, simultaneously displayed images. For example if two images are displayed, each in a respective display window, and the operator moves one image in its display window they may desire the other simultaneously displayed image to move automatically in the same or similar fashion in its display window. Thus, for example, the same or corresponding anatomical features may be displayed at corresponding positions in both display windows.
In another example if a display indicator, such as a pointer, cross-hairs or cursor is displayed on each of two medical images, it may be desired to synchronise movement of the display indicator on each image, relative to the anatomy displayed in each image. Thus, for example anatomy under MPR (Multi-Planar Reformat) cross-hairs may be aligned, scale and rotation of MPR views may be aligned, or anatomy under cursors may be aligned in different images regardless of movement of the images, cross-hairs or cursors.
When the differences between the sets of image data used to produce the different images are sufficiently large, it can be necessary to register the sets of image data to define a spatial (anatomical) relationship between the simultaneously displayed images, so that substantially the same anatomy can be displayed in substantially the same position in each image. Such registration may be required, for example, if the acquisitions of the images are widely separated in time or if there is significant movement of the anatomy between images.
Registration of image data sets can be performed either manually or automatically using known image analysis techniques.
It is preferred that the radiologist or other operator should not have to manually register the images or manually adjust registration while viewing the images as this can add significant time to the interpretation process.
In the case of automatic registration techniques used in synchronous navigation, it is known to apply a transformation obtained from a rigid or affine registration to display indicators, such as cursors, cross-hairs or pointers, in an attempt to ensure that the position of the display indicator is mapped between different simultaneously displayed images and that the movement of the display indicator is uniform between the different images.
A rigid registration in this context may be considered to be a registration in which the co-ordinates of data points in one data set are subject to rotation, translation and/or scaling in order to register the data set to another data set. An affine registration in this context may be considered to be a registration in which the coordinates of data points in one dataset are subject to rotation, translation, scaling and/or shearing in order to register the dataset to another dataset. Thus, a rigid registration may be considered to be a particular type of affine registration.
It is also known to apply registrations obtained from a rigid registration to the synchronised movement of the simultaneously displayed images themselves, such that the relative appearance of the images is rotated, scaled and/or translated. Generally that does not affect diagnostic quality of image as relative proportion of anatomy is not affected.
Rigid registrations can be performed using an average of data points over the full volume in question, or relative to specific fiduciary points or small anatomical region. However, such registrations may be inaccurate away from fiduciary point(s) or specific anatomical regions. Furthermore, the radiologist or other operator will usually have to make several manual adjustments to registration while viewing the images depending on the anatomical region of interest. Manual adjustments to registration are time consuming and lead to an increase in the time required to view the images. Furthermore, there can be significant issues when analysing small structures across multiple time points (for example, lung nodule tracking).
An example of a problem that can arise from use of rigid registration is illustrated schematically when navigating through images derived from volumetric data sets in FIGS. 1a and 1b. 
FIG. 1a shows schematically a series of slices 2a, 2b through a lung volume 4a, 4b obtained from scans at the current time and a prior time. The scans are taken during different breath holds by the patient and, in this case, the lungs are less full at the current time than they were at the prior time. The volumes are registered using a rigid registration. Two of the slices 6a, 6b from the current scan and the prior scan that, according to the rigid registration represent the same region of the lung, are selected for display, as shown in FIG. 1a. 
The user then navigates through the volume using known image processing and navigation techniques, and a further two slices 8a, 8b from the current scan and the prior scan that, according to the rigid registration, represent the same region of the lung, are selected for display. In this case it can be seen that the slice 8a selected from the current scan data is at a position outside the lung. Due to the limitations of the rigid registration, navigation has become out of synchronisation with anatomy. In this case it would be necessary for the operator to make a time-consuming manual adjustment of the registration.
In alternative automatic registration techniques, it is also known to apply a transformation obtained from a non-rigid registration to navigation of images or to display indicators, such as cursors, cross-hairs or pointers, again in an attempt to ensure movement of the display indicator or navigation through the images is synchronised. Such non-rigid registrations usually provide a local deformation field, which is not dependent on fiduciary points or anatomical region. However, generally it can be problematic to apply such non-rigid registrations to images if the images are to be used for diagnostic purposes, as the non-linear mapping can cause a distortion of the images that can interfere with diagnosis. Furthermore, the use of non-rigid registrations to display indicators such as cursors, cross-hairs or pointers can lead to non-uniform navigation between studies and can be confusing or misleading.
Non-rigid registrations include free-form registrations, in which the coordinates of data points in one datasets are subject to a flexible, free-form deformation in order to register the dataset to another dataset. Freeform transformations may be defined using dense vector fields, defining an individual displacement for each voxel in a three-dimensional data set. Freeform transformations may also be defined using other fields or functions, for example using B spline functions or thin plate spline functions.
An example of a problem that can arise from use of non-rigid registration is illustrated schematically when navigating through images derived from volumetric data sets in FIGS. 2a and 2b. 
FIG. 2a shows schematically a series of slices 12a, 12b through a lung volume 14a, 14b obtained from scans at the current time and a prior time. The scans are taken during different breath holds by the patient and, as with FIG. 1a, the lungs are less full at the current time than they were at the prior time. The volumes are registered using a non-rigid registration in this case. Two of the slices 16a, 16b from the current scan and the prior scan that, according to the non-rigid registration represent the same region of the lung, are selected for display, as shown in FIG. 2a. 
The user then navigates gradually, slice-by-slice through the volume using known image processing and navigation techniques, until a further two slices 18a, 18b from the current scan and the prior scan that, according to the non-rigid registration, represent the same region of the lung, are displayed. In this case, the use of the non-rigid registration causes non-linear movement and some of the image slices of the prior image data set are skipped as the user navigates through the images, which can be confusing or misleading for the user.